A fuel cell is an electrochemical cell that can continuously convert the chemical energy formed as a result of a reaction between a fuel and an oxidant to electrical energy. Fuel cells are highly efficient, and the emission levels of these cells are considerably below existing standards.
In a fuel cell, a fuel and an oxidant combine electrochemically without combustion to produce the electrical energy. As long as the cell is continuously supplied with fuel and oxidant, electrical power can be obtained. The fuel of choice in a fuel cell is hydrogen. When hydrogen is used as the fuel, air (oxygen) is normally used as the oxidant. There are, however, a number of disadvantages associated with the use of hydrogen as a fuel. Among these disadvantages are the large weight and volume of gas required in fuel storage systems, the loss of a large percentage of stored energy when the hydrogen is liquefacted, and the present high price of clean hydrogen gas. In addition, the existing infrastructure for distribution of hydrogen is inadequate.
Hydrogen for use as fuel in a fuel cell can be produced internally from hydrocarbons such as natural gas, methanol, diesel, gasoline and other fuels. This is typically achieved in a fuel cell system through the use of a reformer, which reforms hydrocarbon feedstocks to produce hydrogen and carbon monoxide. Some fuel cell systems require an additional shift reaction to convert the carbon monoxide, which can be detrimental to the functioning of the fuel cell, to carbon dioxide.
A typical schematic example of a fuel cell/reformer system is shown in FIG. 1.
The advantage of using hydrocarbons as a fuel for fuel cell systems is that no requirement for hydrogen per se to be transported or stored exists, as hydrogen is internally produced. Most hydrocarbon distribution networks are already in place and consumers are familiar with the handling of hydrocarbons such as natural gas, town gas, diesel and gasoline. Natural gas can, for instance, be supplied directly to the reformer of a stationary fuel cell system and used to generate on-site electricity. Liquid hydrocarbon fuels such as diesel and gasoline would be even more advantageous, as their transporting, storage, and handling characteristics are the same as for conventional diesel or gasoline fueled vehicles.
However, there is a disadvantage when using hydrocarbon sources such as natural gas, gas derived from coal sources (e.g. synthesis gas) and petroleum liquid fuels (e.g. diesel). This disadvantage is that these hydrocarbon sources often contain high levels of sulphur . Sulphur is known to poison both the reforming catalyst and the fuel cell itself.
Typical values of sulphur levels in more environmentally friendly liquid hydrocarbons are shown in Table 1.
TABLE 1 ______________________________________ Sulphur levels occurring in liquid hydrocarbon fuels Fuel Sulphur level (ppm) ______________________________________ Unleaded reference 100 California reformulated gasoline 33 85% ethanol 15% gasoline 8 ______________________________________
According to U.S. Pat. No. 5,589,285, the performance of the co-fired or solid oxide, electrolyte fuel cells degrade considerably when used in a process with sulphur bearing fuels, even at concentrations as low as 1 part per million (ppm). According to a technical report from Westinghouse Electric Company to the Department of Energy, Report No. DOE/MC/22046-2371, sulphur-bearing fuels in solid oxide fuel cells were tested at levels of 2, 10, 25 and 50 ppm. The tests showed that the presence of these levels of sulphur resulted in the degradation of the fuel cell. In an attempt to prevent or lessen the sulphur poisoning of fuel cells, U.S. Pat. No. 5,686,196 teaches a process for the desulphurization of diesel fuels to values less than 1 ppm, prior to the reforming stage to prevent poisoning of the reforming catalyst.
Federal Specification VV-F-800D limits the sulphur content of diesel fuel to between 0.25 and 0.5 weight percent. Some of the more stringent requirements, like those imposed by the Southern California Air Quality Management District and Air Resource Board, currently limit the amount of sulphur in diesel to 0.05 weight percent.
These prior art examples demonstrate that if commercially available hydrocarbons such as natural gas and petroleum-derived liquids fuels, e.g. diesel, were to be used in a fuel cell system, a hydrodesulphurizing step would be required to remove the sulphur. Alternatively, a reforming system and fuel cell would have to be developed that would be tolerant to sulphur in the fuel.